With the exponential growth of wireless communication, new techniques are needed to handle the high capacity of voice and data carried over wireless communication networks. Long Term Evolution (LTE) is a promising network proposal to meet the challenge of increased traffic. It is noted that the terminology “LTE” is not universal. “LTE” as used herein is as a broad term that, depending on the context, may include the meanings E-UTRA (Evolved Universal Terrestrial Radio Access), E-UTRAN (Evolved Universal Terrestrial Radio Access Network) and SAE (System Architecture Evolution). LTE is sometimes referred to as LTE/SAE. More information on LTE can be found in Rumney, LTE and the Evolution of 4G Wireless, John Wiley,© 2009, and Sesia, LTE: The UMTS Long Term Evolution, Wiley© 2009, and the standard documents for E-UTRA: 3GPP TS 36.211: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation;” 3GPP TS 36.212: “Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding;” 3GPP TS 36.213: “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures” the disclosures of which are incorporated by reference herein.
LTE uses orthogonal frequency division multiplexing (OFDM) for radio frequency transmissions in the downlink between a transmitter such as a base station and a user/receiver such as user equipment (UE) (e.g., mobile communication devices such as cell phones, etc.). In the uplink transmission between the user equipment and the base station (typically referred to as “evolved node B” or eNode-B in LTE terminology) through the physical uplink shared channel (PUSCH), a special type of modulation method which is termed single carrier frequency division multiple access (SC-FDMA) is used. Similar to OFDM, SC-FDMA signals carry different orthogonal frequencies, also known as subcarriers. But in contrast to OFDM, the Discrete Fourier Transform (DFT) of the constellation symbols, rather than the constellation symbols themselves, are sent over the subcarriers. As a result, the constellation symbols are sequentially sent in the time domain, and there is a lower peak-to-average power ratio resulting in lower backoff from peak power to achieve higher power efficiency of the high power amplifier of the transmitter.
In the downlink, the data payload is carried by transport blocks which are encoded into codewords which are sent over the downlink physical data channel called Physical Downlink Shared Channel (PDSCH). The scheduling information of the PDSCH codeword(s), including its resource allocation in the subframe and its modulation and coding scheme, is included in the physical control channel, called Physical Downlink Control Channel (PDCCH) [36.212]. Generally, the receiving user equipment decodes the messages in PDCCH and in case it finds that a PDSCH has been assigned to it, it will decode the PDSCH codeword(s) according to the scheduling information decoded from the PDCCH.
In order to prevent the loss of transport blocks, LTE has adopted the Hybrid Automatic Repeat Request (HARQ) scheme. When a base station/eNode-B sends PDSCH to user equipment (downlink transmission) through PDSCH, the data packets are sent together with indicators in PDCCH in the same subframe that inform the receiving user equipment about the scheduling of the PDSCH, including the transmission time and other characteristics of the transmitted data. For each PDSCH codeword that the user equipment receives from the base station/eNode-B, the user equipment responds in the uplink transmission with an acknowledgement (ACK), when the codeword is successfully decoded (indicated by that the CRC check as to the CRC attached with the payload is passed), or a negative acknowledgement (NACK), when the codeword is not successfully decoded (indicated by that the CRC check as to the CRC attached with the payload is not passed). Upon receiving a NACK from the user equipment, eNode-B may choose to retransmit the transport block or skip it. Such mechanism can enhance the system throughput by retransmitting the lost transport blocks in the downlink.
If the user equipment is also transmitting data through PUSCH, the ACK/NACK has to be embedded into the PUSCH. The ACK/NACK information will be encoded and punctured into the data symbols, pursuant to the process prescribed in Section 5.2.2.6 of 36.212. In other words, some of the data symbols are replaced by the ACK/NACK coded symbols. Section 5.2.2.8 of 36.212 prescribes which of the data symbols of the transport blocks are replaced by ACK/NACK symbols, and which are not, derived upon some configurations communicated between UE and the base station/eNode-B, prior to the uplink transmission of the ACK/NACK signals. For the purpose of illustration, we call the positions of those replaced data symbols as “predetermined positions” hereafter.
The channel decoder for the PUSCH codeword and that for the ACK/NACK information are different. Generally, the channel decoder for the PUSCH codeword shall disregard these ACK/NACK coded symbols when decoding the data. And the channel decoder for the ACK/NACK information (“ACK/NACK decoder” hereinafter) shall extract the ACK/NACK symbols only for decoding. Since the PUSCH codeword contains redundancy, it can generally be able to decode with insignificant degradation in robustness, even if some of its data symbols are punctured out.
However, a problem arises when the user equipment is not aware of even the presence of the PDSCH assigned to it, if it fails to decode the PDCCH successfully. In this case the user equipment will not generate ACK/NACK information and its PUSCH will contain data only. This situation has been well recognized and the user equipment response in such case is termed discontinuous transmission (DTX), that is, neither an ACK nor a NACK is transmitted to the base station. Since the eNode-B has no prior knowledge of whether the user equipment fails to detect the PDCCH, it expects that the symbols of the predetermined positions are ACK/NACK symbols and extract them for the ACK/NACK decoder to decode. If the eNode-B receiver disregards the possibility of DTX, either an ACK or NACK would be returned to higher layer, upon the decoding of the extracted symbols which are data in fact, by the ACK/NACK decoder. In general, both ACK and NACK would be equally likely to be returned. If NACK is returned, the eNode-B would assume the PDSCH codeword was not decoded successfully and perform retransmission if appropriate. This would not cause serious problem since the higher layer at eNode-B would still recognize that the PDSCH codeword was actually lost and perform retransmission if it thinks appropriate. A more serious problem arises only when ACK is returned instead, so that the higher layer assumes that the PDSCH codeword has been successfully decoded and skips the retransmission, leading to permanent loss of the transport block.
Hence the consequence of wrongly detecting a DTX as an ACK (“DTX-to-ACK event” hereafter) is more adverse to the system performance than wrongly detecting a DTX into a NACK (“DTX-to-NACK event” hereafter) and is desirable to take more conservative approach in returning an ACK, in order to maintain the DTX-to-ACK probability, which is also referred to as the false alarm rate (FAR). Under this rationale, Section 8.2.4 of 36.104 and Section 8.2.3 of 36.141 prescribe the requirement that the DTX-to-ACK probability should not exceed 0.01, while maintaining certain performance in detection of a true ACK under certain conditions.
Hence, the eNode-B receiver should be so designed that it can, upon the reception of the PUSCH, detect the DTX event and return DTX or NACK to the higher layer, or return NACK to higher layer without explicitly distinguishing whether it is DTX, when DTX event occurs, in order to maintain the DTX-to-ACK probability at a target level. To do so, the base station/eNode-B has to overcome the difficulties arising from reasons such as noise and multipath fading in the wireless communication system.
A common approach to design such receiver is to adopt a threshold-based algorithm for determination of an ACK/NACK signal. That is, in general, the output of the soft channel decoder for the ACK/NACK information is compared with one or more thresholds. These thresholds partition the range of output into multiple intervals. Whether ACK, NACK or DTX is determined, it depends on the interval the value or the magnitude falls into, and this in turn depends on the actual design of the decoder. The challenge of these methods is to design the computation of these thresholds. Existing methods for computing a threshold to achieve a target false alarm rate include those which are implemented in the Constant False Alarm Rate (CFA) detector, a dynamic threshold detector proposed by Philips, and a selective threshold ACK/NACK detector proposed by Huawei Technologies. All of these methods relate to the use of a channel estimate and a noise/signal-to-interference-plus-noise ratio (noise/SINR) from reference symbols in an SC-FDMA frame. Thus the channel estimator and/or the noise estimator are used for determining whether an ACK or NACK transmission has been made from the reference signal.
However, the SINR estimate per subframe is not very precise, particularly in the low SNR regime. Also it generally does not take into account the channel estimation error. Hence the robustness to channel variations of the resulting threshold determination is undermined and a higher margin in the threshold is needed to comply with a given error requirement, thus increasing the misdetection rate.
U.S. Patent Publication 2006/0133290 is directed to improving ACK detection in a mobile terminal by estimating the probability of a discontinuous transmission and then calculating a minimum acknowledgement signal threshold for the mobile terminal using the estimated probability. A detected signal is determined to be an ACK signal or not based on the minimum acknowledgement threshold.
All of the existing methods as previously described for computing the ACK/NACK threshold utilize the channel and noise estimates based on the reference signal symbols to generate the threshold value. However, there are certain drawbacks in using the above estimates. For example, the computed threshold and the predetermined positions of the ACK/NACK signals which will be compared with that computed threshold will be from different stages of the decoding process at the base station/eNode-B, and so the amount of error that has been introduced into the computation of the threshold and the predetermined positions of the ACK/NACK signal would be different and hence it may not be appropriate to compare the computed threshold directly with the predetermined positions for the ACK/NACK signals. Additionally, there are only two reference signal symbols out of the fourteen SC-FDMA symbols in each SC-FDMA subframe; the remaining symbols are data-containing symbols (possibly also containing ACK/NACK signals). Since only the two reference signal symbols can be used in the existing threshold computation methods, the amount of tradeoff available between the complexity and the accuracy of the threshold computation is limited. Moreover the accuracy of the threshold computation is also limited due to the small number of reference signal symbols that can be used.
Thus, there remains a need in the art for improved ACK-NACK detection in LTE communication systems.